README能否添加安装说明

[takakawa]

No description provided.

[shoppWeb]

一.需要网络才能安装
bin/logstash-plugin install --no-verify --local 插件项目根目录/logstash-filter-ipip-0.3.0.gem

二.离线安装
1.制作zip压缩包
zip压缩包目录结构如下:
logstash-filter-ipip.zip
--logstash-filter-ipip-0.3.0.gem
2.安装插件
./bin/logstash-plugin install file:///tmp/logstash-filter-ipip.zip

一般服务器都是局域网环境，第二种方案比较适合。有不对的地方还望指正

[Z10N0110]

I have made a pull request, if the maintainer has not merged it, you can visit my fork.